1. Field of the Invention
The present invention relates generally to systems for reducing turbulent drag associated with fluids flowing through conduits. In another aspect, the invention concerns the delivery of drag-reducing agents to subsea flowlines via a relatively small diameter conduit of an umbilical line.
2. Description of the Prior Art
In the subsea production of oil and gas, production piping typically presents a significant bottleneck because of the difficulty and expense associated with the subsea installation of the piping. The production decrease caused by bottlenecking at subsea flowlines can have severe economic ramifications due to the resulting inability to run the hydrocarbon production system at full capacity. Several options that exist for preventing or curing bottlenecking at subsea flowlines include increasing the diameter of the flowlines, increasing the number of flowlines, or reducing the amount of friction loss in the flowlines to thereby allow more flow through the same diameter lines. The first two de-bottlenecking options of increasing the size or number of flowlines are obviously very expensive. Thus, it is highly desirable to be able to reduce friction losses in subsea flowlines.
It is commonly known that a variety of drag reducers are available for reducing the friction loss of a fluid being transported through a conduit in a turbulent flow regime. Ultra-high molecular weight polymers are known to function well as drag reducers; however, drag reducers vary in their effectiveness. Traditionally, the more effective drag reducing additives have been those containing higher molecular weight polymers. Increasing the molecular weight of the polymer generally increases the percent drag reduction obtained, with the limitation that the polymer must be capable of dissolving in the liquid in which friction loss is affected.
Many offshore oil and gas production facilities are operated from remote locations which can be miles away from the production wells. When remote facilities are used to operate a subsea production facility, an umbilical line is typically employed to provide power and various flow assurance chemicals to the production facility. Such umbilical lines generally include a plurality of relatively small-diameter injection lines through which various chemicals can be introduced into the flowline at an injection point proximate the production wells. These chemicals generally include low-viscosity fluids such as hydrate inhibitors, wax inhibitors, and corrosion inhibitors which help to improve flow conditions in the flowline.
In the past, it has been proposed that drag reducing agents could be transported through an umbilical line to thereby affect a reduction in friction loss in the subsea flowline. However, due to the high viscosity and/or large particle size associated with commercially available drag reducers, existing drag reducers cannot be transported through the relatively small diameter conduits an umbilical line without causing plugging or unacceptable pressure drop. Methods have been devised for transporting a high-viscosity, high-polymer-content drag reducer through a chemical injection line of an umbilical by facilitating the flow of the drag reducer with an immiscible low-viscosity liquid material injected at the periphery of the chemical injection line. However, this method requires special equipment for introducing the low-viscosity material into the periphery of the chemical injection line. Furthermore, these methods do not address the problems associated with drag reducers that require the formation of a strand (described below) to effectively dissolve in the host fluid.
Commercially available gel drag reducers are typically highly viscous (e.g., greater than 10,000 cP or sometimes greater than 100,000 cP at typical pumping shear rates) and highly concentrated with ultra-high molecular weight polymers. Even at polymer concentrations as low as 5%, these gel drag reducers are still highly viscous.
In the past, when drag reduction has been needed in chaotic flow environments, it has been necessary to utilize suspension or slurry drag reducers. However, conventional suspension or slurry drag reducers typically contain solid particulate matter that is too large to flow through an umbilical line without plugging the line. Furthermore, highly viscous materials present difficulties in transport across long umbilical lines because of the tremendous pressure drop associated therewith.